Porous metal articles having a predetermined pore character

ABSTRACT

A porous metal article having a predetermined pore structure. The porosity is provided by the use of an extractable particulate in a powder forming route to create a desired porosity. Extraction of the pore forming particulate prior to sintering of the powder minimizes contamination of the sintered article and allows for the processing of material sensitive to contamination such as titanium. Added functionality can be gained by co-forming the porous material with non-porous material to create an article with layers of differing characteristics. The article is suitable for use as an implant body is porous enough to facilitate tissue in-growth and bony fusion.

BACKGROUND OF THE INVENTION

This invention relates to porous metal articles having tailored porosityand methods of manufacturing such articles by using extractableparticulates.

Porous metal articles are used in many applications including orthopedicimplants, bone growth media, filters, sound suppression materials, fuelcells, catalyst supports, and magnetic shielding. Such porous articlesmay have open or closed porosity as well as a wide range of pore size,shape, density, and distribution. The specific structures and propertiesrequired depend on the application. Known methods of manufacturingporous metals include formation by foaming, diffusion bonding orsintering of powders, depositing a material upon a porous substrate, useof vaporizing materials, and plasma etching.

Foaming describes the creation of porosity through the introduction ofsome agent, organic or inorganic, that creates voids during the formingprocess. Diffusion bonding or sintering processes create partially densenetworks of powder particles and pores. Porous metal can be formed bythe deposition of the desired metal onto a foam or substrate material.This deposition may be accomplished by a number of methods includingdipping the substrate material in slurry containing metal powder,evaporation and condensation of the material on the substrate.

When using a vaporizing material as a void former in metals, thematerial is removed thermally, and the metal matrix material can easilybecome contaminated by the vaporizing material when the article isexposed to the necessary high processing temperatures. The evolution ofthe products of decomposition and the vaporized materials are difficultto control which reduces the robustness of the process, thus limitingprocessing capabilities to articles having smaller cross-sections.Complications resulting from oxidation and other contamination inthermal decomposition or sublimation approaches make such processes notsuitable for high-purity applications such as bone implants. Inaddition, the heat and high pressure of the compaction and densificationprocesses will deform the void former material, thereby not allowing thetailoring of pore properties.

The pores formed by the plasma spray process are limited in thethickness of the pore layer and result in random pore formation. Vapordeposition techniques for making porous tantalum structures results inarticles having a relatively poor bending strength and not suited inmany applications where the article has bending force exerted thereon.

Moreover, techniques depositing a powder onto a foam substrate viaslurry are limited by the slurry's ability to penetrate and evenly coatthe substrate. Other methods such the construction of shapes by thelaser sintering of powders are limited to small section by the inherentexpense and time-intensive nature of the process.

Thus, there remains a need to provide a porous metal with a controllablepore structure which possesses strength and structural integrity and isfree of contamination.

BRIEF SUMMARY OF THE INVENTION

This invention provides a metal article which is at least partiallyporous and exhibits a predetermined pore character.

In one aspect, this invention provides a sintered porous metal articlesuitable for use as an implantable device.

In still a further aspect, this invention provides for the forming ofmetal articles with layers of differing porosity by forming layerscontaining extractable particulate of varying characteristics. Theselayers may also be completely free of the extractable particulate toprovide a dense layer.

In still a further aspect, this invention allows for the forming ofporous metal articles having large cross-sectional areas.

The present invention also discloses methods of manufacturing a porousmetal article comprising the steps of blending a metal powder andextractable particles to form a homogenous mixture, forming a composite,extracting the particles using a fluid to form a metal matrix greenarticle, and sintering the green article. Preferably, the step ofextracting the particles comprises exposing the green shape to a waterbath.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS

For a fuller understanding of the invention, reference is made to thefollowing descriptions taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a schematic illustration of a compacted mixture illustratingthe compacted matrix material and the extractable particles.

FIG. 2 is a schematic illustration of a compacted mixture with theextractable particles partially extracted.

FIG. 3 is a schematic illustration of a metal matrix structure after theextractable particles have been removed.

FIG. 4 represents a micrograph at six times magnification showing across-section of a layered titanium material made in accordance with thepresent invention.

FIG. 5A is a side view of a typical implant in the upper femur in thehuman body.

FIG. 5B is a magnified view of the femur in FIG. 5A and shows in detailthe interface between the implant and the bone.

FIG. 6 represents a micrograph at six times magnification showing thesurface of porous titanium material made in accordance with the presentinvention.

FIG. 7 represents a micrograph at six times magnification showing thesurface of porous titanium material made in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to porous metal articles having porositycharacteristics that are determined by an extractable material which isremoved prior to sintering and methods to manufacture such porous metalarticles.

Powder metallurgy processes are used to form metal articles wherein aportion of the powder being processed is replaced with a pore formingmaterial which is removed prior to sintering to form the desiredporosity. The metal articles include articles that are elemental metal,metal alloys, and metal composites. The pore forming material isreferred to as an extractable particulate or pore-former.

The metal powder and pore-former are mixed, the article is formed, andthe pore-former is extracted. The powder remains to form the metalmatrix of material around the pores formed by the extractableparticulate. The matrix can then be further shaped and sintered to givethe article greater strength.

The use of an extractable particulate to form porosity in an articleformed from powder allows control over the pore properties, includingpore density, size, distribution, and shape. The pore properties in thefinal article are determined primarily by the properties of theextractable particulate and, thereby, are tailored by the selection ofthe extractable particulate. By specifying one or more properties andprocessing the extractable particulate to reflect the desired poreproperties, the present invention allows for the tailoring of the porecharacter.

The concentration or loading of the extractable particulate in the metalpowder determines the porosity density in the final article. The extentof interconnectivity between the pores is varied by the concentration ofthe extractable particulate as well as the size of the pore formingmaterial and the matrix forming powder. For example, a compact madeusing a titanium powder of a −325 mesh as the matrix material and 70percent or more by volume potassium chloride granules having a mesh sizeof −20 to +60 will exhibit continuous interconnectivity in the finalarticle. If the potassium chloride granules are reduced in size,interconnectivity will occur at a lesser volume fraction of porosity.

The required interconnectivity and concentration of extractable materialused is dictated by the application. Some applications, such asosteointegration and filtration require pore interconnectivitythroughout the article. Other applications such as those targeting areduced weight or density may require a closed pore cell structure.Applications desiring a more porous structure use a larger fraction ofextractable material and applications desiring a less porous structureuse a lesser fraction of extractable material.

The difference in volume percent porosity of the initial compactedarticle compared with the sintered article depends on the materials andprocessing, but is generally less than five percent. Changes in thepercent porosity of a compact during processing are due to severalfactors. The primary factor is that the initial mixing of powdersassumes a final density of 100% for the sintered matrix material.However, depending on the subsequent processing and desired finaldensity, sintered densities may vary from 85% to 100% dense. Once thechange in volume percent porosity for a particular metal powder andextractable material process has been determined, it remains constantand thus permits the precise tailoring of porosity volume in the finalarticle.

If the percent content of extractable material is too high, it mayrender the compacted article too fragile to handle between extractionand sintering, or may inhibit compaction of the matrix material.Preferably, the pore-former content will be between 50 and 95 percent.If lower porosity content is desired, it may be advantageous to compactthe material to relatively low density in order to allow the easyextraction of the pore former.

The pore structure and dimensional properties of the final article aretailored by the size and morphology of the extractable particulate. Thesize and shape properties of the extractable particulate depend on thematerial used as the extractable particulate. The shape of a pore (forexample, spherical, angular or irregular) will correspond to the shapeof the extractable particulate used to form it. The size of a specificpore in the final article is proportional to the size of the extractableparticulate that is used to form the pore. The proportionality isdetermined by any shrinkage that may result in the sintering of themetal powder material. The shrinkage encountered during sintering isdictated by the degree to which the matrix is densified. The shrinkageof an article during sintering can be approximated by the followingformula:ΔL/L _(i)=1−(ρ_(i)/ρ_(f))^(1/3)

-   -   Where:        -   ΔL=Change in length.        -   L_(i)=Initial length.        -   ρ_(i)=Initial density of matrix.        -   ρ_(f)=Final density of matrix.

This formula does not address changes in shrinkage due to changes in thechemistry of the matrix material such as carbon loss or oxygen pick-up.These are typically slight discrepancies and the actual results aregenerally within a few percent of the calculated results. Moreover, oncethe actual shrinkage has been determined for a specific matrix materialand manufacturing process, the invention provides for a very consistentand repeatable process with essentially no variability in shrinkage.This allows for the precise control over and ability to tailor the finalpore size and shape.

The desired pore size distribution for the final article is determinedby the particle size distribution of the extractable particulate.Although some shrinkage may be encountered during processing, the finalpore size distribution is directly proportional to the particle sizedistribution of the extractable particulate. This proportionality isdetermined by the shrink percentage as discussed above. Thisrelationship allows for the tailoring of articles with very specificpore size distribution by engineering the particle size distribution ofthe extractable particulate. Methods of tailoring particle sizedistributions are well known and include milling, grinding, sieving, andair classification. Pore size distributions can be manipulated toproduce wide or narrow pore size distributions, as well a bi-modal ormulti-modal distributions.

The pore-former characteristics can be varied and combined to achievethe desired pore character. This may include combining several differentpore-formers to achieve certain unique properties such as a combinationof angular and spherical pores, or a combination of non-continuous poresize distributions.

In addition to considerations related to the predetermination of thedesired porosity, there are several process related considerations to bemade when selecting an extractable particulate. In a preferredembodiment, the extractable particulate should be compatible with anyadditional processing steps, it should not leave undesired residue inthe final part, it should not react with the matrix material, and itshould have adequate strength so as not to be deformed duringprocessing, such as compaction. If the forming route includes elevatedtemperatures prior to particle extraction (for example, if forming isdone via Metal Injection Molding (MIM)), the extractable particulateshould exhibit thermal and mechanical stability at those temperatures.

The extractable particulate material should be removable by extractionvia a fluid prior to sintering. In a preferred embodiment, the removalis done by dissolving and extracting the pore-former in a water-basedsystem. In this system, ionic bonded materials such as metal salts aredesirable as extractable particulates because of ease of removal. Anysalt ions should be completely removed from the metal matrix prior tosintering.

Mechanical stability of the extractable article is important. In apreferred embodiment, the extractable particulate material should beselected so it will not deform and store energy during any processingsteps (for example, compaction) of the metal powder and extractableparticulate mixture. If a material is compressed during the compactionprocess, then upon removal of the compaction force, any energy stored inthe compacted particulate acts upon the compact. This may result in thecompact cracking upon removal from the compaction tool or disintegratingduring the water extraction step.

According to the present invention, the extractable particulates arepreferably water soluble solids, and most preferably potassium chloride,potassium sorbate, sodium chloride, or a mixture thereof.

Some polymeric materials are also suitable for use as extractableparticulates depending on melting behavior and solubility. For example,hydroxypropylmethyl cellulose powder may be used for materials that areprocessed by metal injection molding.

The metal powder used should be chosen based on the desired propertiesof the final product. The specification of the metal powder should beselected to ensure the article performs predictably during processing.Particles having an irregular, angular, or ligamental nature will deformaround or into one another during processing, resulting in better greenstrength, i.e., the strength of the article after forming the metalpowder and extractable material into a shape. For example, when using acompaction technique to form the shape, selecting metal powders withangular characteristics will give the compact adequate green strength.Hydride/dehydride processes for making metal powder result in adesirable angular structure suitable for formation by compaction.

The metal powder is preferably titanium, tantalum, cobalt chrome,niobium, stainless steel, nickel, copper, aluminum, or any alloysthereof However, it will be apparent that this invention may readily beadapted to other metals by appropriate selection of the extractablematerial and processing conditions.

In one example for medical implant devices, angular titanium powder of amesh size between −100 to +635 is be used as the matrix material withpotassium chloride particles between 100 and 2000 micron as theextractable particulate. This system may then be formed by compaction.

In another example, potassium sorbate is used as the extractableparticulate. Granules of potassium sorbate that are between 600 and 1000micron in diameter with 17-4 stainless steel metal powder (−22 mesh) arewell suited for use with a water-soluble stainless steel feedstocksystem for use in metal injection molding.

Once the extractable material and metal powder have been selected, thematerials are then blended. Blending techniques such as V-blending,mixing on a jar mill, hand blending or use of other known powder mixingtechniques can be used.

When the metal powder and the pore-forming material are blended togethercare must be taken to ensure that the materials form a homogenousmixture irrespective of any variation in particle size and density. Inone embodiment, a small amount of a homogenizing aid can be added tohelp the different materials adhere to each other and create ahomogenous mixture. In a preferred embodiment, the homogenization aidcan easily be removed after the metal powder and pore-former arecompacted. In a most preferred embodiment, the homogenizing aid either(a) is removed by the same fluid used to extract the extractableparticulate, or (b) is stable at room temperature and pressure, but willevaporate at elevated temperatures or reduced pressure. Examples of suchhomogenizing aids include polyethylene glycol (PEG), which reducessegregation and is removable in a water bath prior to thermalprocessing, and higher alcohols or isoparaffinic solvents, as well asorganic liquids, such as acetone, which can easily be removed byevaporation prior to or after compaction.

In one example, titanium powder of a mesh size between −170 to +635 isused as the matrix material, with potassium chloride particles between100 and 2000 micron, and eight percent by weight of acetone is added,resulting in a homogeneous mixture after V-blending.

In order to ensure good blending and prevent agglomeration, the mixturemay be sieved between blending steps or after blending. As anotherexample, in a system using potassium chloride granules having a sizerange between 250 and 850 micron and a −325 mesh titanium powder andeight weight percent acetone, the mixture can be sieved through a 1000micron screen to ensure that there are no significant agglomerates.

The homogenizing aid added to the blend to increase homogeneity can beselected to be easily removed before or after the compaction step. Thereare advantages to either approach. The inclusion of a third material towet the pore-former and the matrix material can vastly improvehomogeneity. If this homogenizing aid is left in the blended materialsduring the compaction, it can limit the degree to which the materialsare compacted because it creates a hydrostatic lock at the limit of themixture's compatibility. This results in the compacted matrix beingslightly less dense than it would have been if the material was notpresent. In many cases this difference in density is not critical. Ifhowever, the highest possible compacted density is required then thehomogenizing aid should be removed prior to compaction.

If the homogenization aid remains in the blend during the compactingstep, the amount of material used as an aid should be optimized tominimize its effect on compaction while still providing adequatehomogenization. In a preferred embodiment, polyethylene glycol (PEG)having a molecular weight of 200 is used to increase homogeneity and isremoved after compaction during the water immersion step used to removethe extractable particulate. The amount of PEG 200 used is based on thetheoretical density of the mixture of extractable and matrix material ifthere were no voids present. As an example, a porous titaniumimplantable device can be formed using potassium chloride granuleshaving a size range between 250 and 850 micron with a −325 mesh titaniumpowder and 0.05 grams PEG 200per cubic centimeter of the mixture attheoretical density.

After blending, the metal powder, extractable material, and anyhomogenizing aid is processed to achieve a desired green shape. Thisprocessing can be by known powder consolidation techniques such as diecompaction, isostatic pressing, or metal injection molding.

FIG. 1 shows an extractable particulate and a matrix powder that havebeen blended and then consolidated by compacting the mixture into acylinder. The extractable phase (10) has retained its initial shapeafter die compaction and is surrounded by the angular matrix powder(11).

After the green article is formed, according to the present invention,the extractable particulate is dissolved out of the article bysubjecting the article to an extraction process using a fluid.Dissolving the pore-former in a fluid in which the pore-former issoluble will remove the pore-former from the green article. In apreferred embodiment, a compacted article using a salt as a pore-formeris immersed in water, thereby dissolving and removing the extractableparticles. Other fluids, including gasses and liquids may be useddepending on the dissolution properties of the extractable pore-former.What remains after extraction is an unsintered metal matrix structuresurrounding the pores formed by the extractable material.

When using water to dissolve the extractable particulate, the water bathshould be deoxygenated to reduce or eliminate the potential foroxidation of the metal powder in the water. This can be done by, forexample, bubbling nitrogen through the water, or other knowndeoxygenating means.

FIG. 2 shows a schematic of a compacted mixture in accordance with thepresent invention with the pore-former partially extracted. Thecompacted matrix powder (21) surrounds the remaining extractable phase(20) and the pores (22) formed during the extraction process. The pores(22) retain the dimensional properties of the extractable particulateused to form them.

By further using a rinsing step after immersion, any amount ofpore-former remaining after exposure to the dissolution fluid, can beeliminated or reduced to negligible trace amounts. While certain metalsystems may be less sensitive to contamination than others, it isgenerally desirable to reduce any extractable material remaining in thecompacted article to below 2000 parts per million (ppm), and inapplications such as medical implants, preferably to below 500 ppm, andmost preferably below 200 ppm. As an example, a titanium porous articleis made using potassium chloride as the pore-former and has a residualchlorine content in the green article of less than 25 ppm and apotassium content of less than 50 ppm. Such articles will have nosignificant contamination as a result of the pore-former and aresuitable for orthopedic implant applications.

The amount of pore-former extracted can be monitored by known means foranalyzing the fluid. As an example, if water is used as the extractionfluid, the amount of extractable material in the water can be measuredin ppm by a conductivity meter. Reduction of weight of the green articlecan also be used to monitor removal of the extractable material.

FIG. 3 represents a schematic of an unsintered metal matrix structureafter the pore-former has been removed. The compacted metal matrix (30)defines the porous region (31) formed by the removal of the extractableparticulate. The article has adequate green strength to allow handling.

The present invention also provides for the processing of material withcross-sectional areas greater than one inch. The removal of theextractable material can be determined by monitoring the extractionfluid, thereby allowing complete removal prior to placing the parts inthe sintering furnace.

The present invention further provides an economic processing systemwith no or low environmental impact. When a water extraction approach isused, the water baths are inexpensive to purchase and operate. Afterprocessing, the water can be separated from the solute by evaporation orprecipitation, allowing for recycling or recovery of the pore-former.Materials preferably used as water soluble pore-formers such aspotassium chloride or potassium sorbate do not pose and special handlingor safety concerns.

The extraction of the pore-forming material prior to the article beingexposed to higher temperatures eliminates the contamination of thematrix material by the pore-former. The mechanical properties andapplications of the porous metal dictate the level of contaminationpermissible in the material. The mechanical properties of titanium andits alloys are greatly reduced by the presence of oxygen, carbon,nitrogen or hydrogen, and titanium becomes increasingly reactive astemperature increases. In an embodiment where a reactive metal is usedas the matrix material, it is desirable to remove more than 99.90percent of the pore-former at low temperatures, typically below 100° C.,through fluid extraction.

After extraction of the pore former, the metal powder matrix structurecan be sintered by known means to densify the matrix material. Sinteringconditions are determined by the properties of the powder beingsintered. The times, temperatures, pressures and atmospheres used in asintering cycle are selected based on the nature of the material beingsintered. Sintering of metal powders is a well understood field and theselection of a sintering cycle can be made by those skilled in the art.Depending on desired properties of the final porous article, thematerial may be subjected to post sintering processes such as hotisostatic pressing (HIP) to further densify the article.

The invention may be understood with reference to the followingadditional examples, it being understood that all of the examples arepresented for the purpose of illustration only and are not intended tobe limiting of this invention, the true nature and scope of which areset forth in the claims below:

In Example 1, angular titanium powder formed via a hydride/dehydrideprocess with a particle size below 270 mesh is blended with potassiumsorbate granules having a particle size between 600 and 1000 micron. Ona volume fraction basis the mixture contains 15 percent titanium powderand 85 percent extractable particulate. The ratio of matrix material andextractable material can be tailored to the desired volume percentporosity. Eight percent by weight acetone is added. This mixture isblended together with gentle mixing using a turbula. This mixture tothen poured into a rubber tool approximately 1.5 inches by 3 inches,which serves as a cold isostatic press (CIP) tool. Once poured into thetool, the mixture is dried in air at 50° C. for one hour to evaporatethe acetone. The time required to remove the acetone is based on thesize of the part. After the acetone is removed, the tool is sealed andplaced in the CIP. The mixture is compacted to 40 ksi. The compact isremoved from the tool and placed in a water bath between 30 and 60° C.After the bulk of the potassium sorbate has been removed, the water isreplaced several times to remove any traces of the extractableparticulate. A conductivity meter is used to determine the completenessof the removal of the potassium sorbate. The water bath is rinsed untilthe dissolved solids in the water are at or below 25 ppm. After removalfrom the water bath the compact is dried to remove any remaining water.This is done in air between 20 and 60° C. The porous compact is thenplaced in a sintering furnace and heated at a rate of 10° C. per minuteto 1255° C. for three hours in a partial pressure of argon. The sinteredcompact has a pore character determined by the pore-former, and thematrix material has a density of about 95 percent of theoretical. FIG. 6is a micrograph of porous titanium made using this process with aporosity of 85 volume percent. The sintered metal titanium (61) definesthe pores (62).

In Example 2, tantalum powder having a particle size below 200 mesh iscombined with potassium chloride having a particle size between 250 and850 micron. On a volume fraction basis the mixture contains 15 percentmatrix and 85 percent extractable particulate. This mixture is combinedby gentle mixing using a V-blender. Six percent by weight acetone isadded to the blend to prevent segregation of the two components. Thismixture is then poured into rubber tool, approximately 0.375 inches by1.5 inches, which serves as a cold isostatic press (CIP) tool. Oncepoured into the tool the mixture is dried in air at 50° C. to evaporatethe acetone. After the acetone is removed, the tool is sealed and placedin the CIP. The mixture is compacted to 40 ksi. The compact is removedfrom the tool and placed in a water bath between 30 and 60° C. Afterapproximately 75 minutes, the bulk of the potassium chloride has beenremoved and the water is replaced several times to remove any traces ofthe extractable particulate. A conductivity meter is used to monitor theextraction of the potassium chloride. After removal from the water baththe compact is dried in air at a temperature between 20 and 60° C. toremove any remaining water. The porous compact is then placed in asintering furnace and heated at a rate of 10° C./min to 1300° C. for 1hour in a partial pressure of argon. The sintered compact has a porecharacter determined by the pore-former.

In Example 3, angular titanium powder typically formed via ahydride/dehydride process and having a particle size below 270 mesh isblended with potassium chloride granules having a particle size between20 and 60 mesh. On a volume fraction basis the mixture contains 15percent matrix and 85 percent extractable particulate. The ratio ofmatrix material and extractable material can be tailored to the desiredvolume percent porosity. This mixture is blended together with gentlemixing. 0.05 g per cc of mixture PEG 200 is added to the blend toprevent segregation of the two different components. This mixture issieved in a 1000 micron sieve to eliminate any agglomeration. Themixture is then poured into a cold isocratic pressing tool in the shapeof a rectangle with the following dimensions: two inches by three inchesby five inches. The mixture is compacted at 38 ksi. The compact isremoved from the tool and placed in a deoxygenated five gallon waterbath between 30 and 60° C. The water is replaced once per hour for thefirst 6 hours. After an additional 12 hours, the water is replacedagain. After a total of 20 hours the water bath will have less than 25ppm of the dissolved pore-former and the green article may be removedfrom the water bath. A conductivity meter is used to monitor theextraction of the potassium chloride. After removal from the water baththe compact is dried in air at a temperature between 40 and 50° C. forthree hours to remove any remaining water. The porous compact is thenplaced in a sintering furnace and heated at a rate of 10° C. per minuteto 1300° C. for three hours in a partial pressure of argon. The sinteredarticle has a pore character determined by the pore-former. Duringsintering the article exhibits a linear shrinkage of 16.5 percent. FIG.7 is a micrograph of porous titanium made using this process. The parthas a porosity of 85 volume percent. The sintered metal titanium (71)defines the pores (72).

The present invention includes articles having multiple pore characterssuch as layers of differing porosity or porous layers on densesubstrates. The forming of articles with non-homogeneous porosity can beaccomplished simultaneously or sequentially. An example of asimultaneous route is the die compaction or cold isostatic pressing ofmultiple layers of material at the same time, where each layer has apore-former with differing granule properties. This example may alsoinclude a layer of metal powder without any extractable material,resulting in a layer of high density. An example of a sequential routeis the compaction of a metal powder and extractable particulate mixtureonto a solid metal part or substrate. Another example of a sequentialroute is to take a previously compacted article and compacting a layeronto it. As used herein, the term layer includes any section of materialhaving properties that differ from those surrounding it.

A layer structure according to the present invention has advantages inmany applications. A non-porous layer may allow easy fixation of theporous material to a substrate. The non-porous layer may also serve asbarrier to isolate the porous material. The non-porous layer can alsoserve as a mating surface in a thermal management device, and the porousmaterial as a high surface area heat dissipating device. The porousmaterial may serve as kinetic energy dissipating element and thenon-porous layer serve to distribute the force of an impact across theporous section.

An article may be formed having layers of differing porosity, providinga part that has a porosity gradient or differing pore character in anarticle. By co-forming materials with layers containing varying amountsof extractable particulate, an article having areas of differingporosity can be formed. Included in this approach is the forming of alayer that is essentially free of pores. In this manner an article withporous areas on the outside and dense areas on the inside could beformed. Alternatively, an article that is porous on the inside and denseon the outside may also be formed.

FIG. 4 shows an example of an article with layers of varying porositymanufactured according to the present invention. FIG. 4 is across-section of an article having a layered nature with alternatingsection of differing predetermined porosity. A first region (40) has a85 volume percent porosity and pore sizes in the range of 200-850microns, a second region (41) is a dense layer which has been sinteredto a closed porosity of approximately 95 percent dense.

The article in FIG. 4 is formed by first adding titanium powder of a−325 mesh to a conventional die compaction tool and compacting thematerial gently so as to distribute it evenly around the tool withoutsignificant densification. Then a mixture of 15 volume percent −325 meshcommercially pure titanium metal powder and 85 volume percent −20 meshpotassium chloride with 0.05 g per cc of mixture PEG 200 as ahomogenizing aid is added to the tool and compacted at 38 ksi. Theresulting compact has a defined layer of porous material and non-porousmaterial and is processed by removing the extractable phase by immersingthe part in water at 60° C., drying the article and sintering in apartial pressure of argon by heating at a rate of 10° C./min to 1255° C.and soaking for two hours. The highly porous region (40) is formed bythe mixture of titanium powder and extractable particulate. The secondregion (41) is of higher density titanium formed by the layer oftitanium powder without any extractable particulate.

It is also desirable to form a porous surface onto a preexistingstructure or substrate. An example of this is an orthopedic implant. Thebulk of the implant is a solid piece that provides the strength of theimplant. Porous areas are desired on this piece at areas where boneingrowth is desired. In one example, a mixture of metal powder andextractable particulate is cold isostatically pressed onto an implantbody in the areas where ingrowth is desired.

An example of an orthopedic implant is shown in FIG. 5A. The implantincludes the solid base (50) with a porous outer surface (51). Theimplant is inserted in the femur bone (53).

FIG. 5B shows is a detailed view of the interface between the implantand the bone. The bone has grown into the open pores (52) of the porousouter surface (51) of the implant. The bone (53) is thereby structurallyaffixed to the implant (50). This compact having layers of porousmaterial and non-porous material is formed by placing the solid stem ofthe implant into a rubber tool designed to hold it and also allow themixture of metal powder and extractable particulate to be poured into agap around the areas on the stem where a porous area is desired. Then amixture of 20 volume percent −325 mesh commercially pure titanium metalpowder and 80 volume percent −20 mesh potassium chloride with 0.05 g percc of mixture PEG 200 as a homogenizing aid is poured into the tool andtapped to assure proper filling. The tool is then sealed and coldisostatically pressed at 40 ksi. The resulting article has a definedlayer of porous material and non-porous material. The compacted articleis removed from the tool and placed in a water bath at 40° C. to removethe extractable phase. After the removal is complete the part is driedand sintered in a partial pressure of argon at a heating rate of 10°C./min to 1290° C. and held for 1 hour.

When a layer is compacted onto a substrate the close mechanicalinterface created allows the matrix material to diffusion bond with thesubstrate material at elevated temperatures. This can typically be donein the same temperature regime used to sinter the matrix material.

When compacting a layer onto a substrate, the layer does not have to bethe same material as the substrate. However the materials should beselected to be compatible with the subsequent processes of sintering thematrix and diffusion bonding the layer to substrate the substrate.

In another example, a 3″ diameter hemisphere composed of Ti-6Al-4V andhaving a thickness of 0.25 inches serves as the substrate and is placedin a rubber CIP tool that allows for a 0.25 gap between the hemispheresurface and the tool. The surface of hemisphere that will contact theporous layer has been previously roughened to create a interlock betweenthe substrate and the layer. A mixture of 20 volume percent −325 meshcommercially pure titanium metal powder and 80 volume percent −20 meshpotassium chloride is poured into the tool filling the gap between thesubstrate surface and the tool, covering the substrate to a thickness of0.25 inch. The tool is sealed and cold isostatically pressed. Thecompacted article is then immersed in water between 50 and 60° C. toremove the extractable particulates, and, thereafter, dried. The articleis placed in a vacuum furnace at a partial pressure of argon and heatedat 10° C./min to 1255° C. and held for two hours. The resulting articlehas a hemispherical surface that is covered with a layer of titaniumhaving porosity of 80 volume percent and a thickness of 0.2 inches.

The present invention also includes large porous shapes from which adesired shape can be formed using conventional metal forming methods.This technique of forming a desired porosity can also be applied to manybinder assisted forming routes such as powder injection molding,extrusion or casting. The specific nature of the binder and formingroute must be considered in the selection of an extractable particulate.

For example, in the field of powder injection molding, the binder usedto form the powder article is typically removed in two stages. The firststage is typically an extraction stage where a binder phase is removedby solvent extraction, catalytic decomposition, or evaporation. Theextractable particulate may be selected to be compatible with the firststage extraction process, eliminating the need for an additional step.For example, if the first stage debinding is done by using water as asolvent, than the extractable particulate can be selected to bewater-soluble.

Powder injection molding compounds typically have two components, thepowder system and the binder system. The powder system contains thepowders that are to be formed and sintered. The binder system is usuallycomposed of various polymers and waxes to allow the forming of thedesired shape. The extractable particulate is added to the powder systemto create the desired porosity.

The extractable particulate can be combined with commercially availablebinders. For example, a 17-4 stainless steel article having porosity of70 percent by volume can be formed by using powder system of thecomposition:

-   70% Potassium chloride-   20% 17-4 master alloy powder, with a particle size less than 20    micron-   10% Carbonyl iron powder    This powder system can be combined with a commercially available    water soluble binder system such as F566 Binder System manufactured    by Praxis Technology, Glens Falls, N.Y. The mixing of the materials    may be done sequentially to allow proper mixing of the metal powders    and the binder prior to adding the extractable particulate. For    example, in the above powder system, the 17-4 master alloy powder    and the carbonyl iron powder are combined with binder system and    mixed until the binder system has melted and a homogeneous mixture    is achieved, known as feedstock. The extractable particulate can be    added to the feedstock and mixed until homogeneous. The feedstock    would have a composition of 92 percent by weight powder (extractable    and matrix forming) and eight percent binder.

The necessity for sequential mixing is dictated by the specific bindersystem used. For instance, the binder system may incorporate emulsifiedsurfactants which contain water that is evaporated during mixing. If theextractable particulate is water soluble, it should be added after thewater has been removed from the mixture.

In the preceding example, after the feedstock is mixed, it is injectionmolded to form the article. The injection molding conditions are basedupon the specific binder system used. Using a 17-4 master alloy powder,the melt temperature would be 180-190° C. and the molding pressure about800 psi. After the article is formed, it is immersed in water at 65-75°C. The use of a water soluble binder system and a water solubleextractable particulate allow the removal of the extractable particulateand the water soluble phase of the binder in the same step. The lengthof immersion is based upon the size of the molded part. It is helpful toadd clean water and removed used water from the bath to maintain anadvantageous concentration gradient between the internal section of thepart and the water bath. After removal of the bulk of the extractablematerial, the parts can be rinsed to remove any residual traces of theextractable material.

Following the removal of the extractable particulate the article isprocessed in a manner consistent with the known art of powder injectionmolding. In formulations containing a high porosity it may be helpful topresinter the parts in air to allow for strengthening of theinterparticle bonds. Oxidation of the carbonyl iron will help to retainthe pore structure after the remainder of the binder melts or isthermally decomposed. In the case where air presintering is notfeasible, the selection of a powder system using non-spherical powdersis helpful in retaining the pore character.

The nature of this invention can be applied to other binder systems usedin the binder assisted forming of sinterable powders. The field ofbinder assisted forming encompasses many different forming techniquessuch as the injection molding, compression molding, compaction,extrusion, or green machining of articles comprised of a powder andbinder mixture.

Although the present invention has been described in terms of examplesand presently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe claims be interpreted as covering all alterations and modificationsas fall within the true spirit and scope of the invention.

1. A method of making a porous metal article, the method comprising: a.blending a metal powder and an extractable particulate to form acomposite; b. compacting the composite into a green article by applyingpressure in excess of 5,000 pounds-per-square inch to deform particlesof the metal powder; c. removing the extractable particulate from thegreen article to form a metal matrix and pore structure; and d.sintering the metal matrix and pore structure.
 2. The method of claim 1,further comprising adding a homogenizing aid to the composite.
 3. Themethod of claim 2, wherein the homogenizing aid is water soluble.
 4. Themethod of claim 2, wherein the homogenizing aid is polyethylene glycol.5. The method of claim 1, wherein the metal powder is selected from thegroup consisting of: titanium, tantalum, cobalt chrome, niobium,stainless steel, nickel, copper, aluminum, and alloys thereof.
 6. Themethod of claim 5, wherein the metal powder is −100 mesh.
 7. The methodof claim 1, wherein the extractable particulate is selected from thegroup consisting of: salts, sorbates, and inorganic materials.
 8. Themethod of claim 1, wherein the extractable particulate has a diameterbetween about 100 and 2000 micron.
 9. The method of claim 1, wherein theextractable particulate is removed by dissolving the extractableparticulates in a fluid.
 10. The method of claim 9, wherein the fluid iswater.
 11. The method of claim 9, wherein the fluid is a gas.
 12. Themethod of claim 1, wherein the metal matrix and pore structure hassufficient green strength to be handled prior to sintering.
 13. Themethod of claim 1, wherein the porous metal article is selected from thegroup consisting of: orthopedic implants, and filters.
 14. The method ofclaim 1, wherein the porous metal article is further processed to format least part of a device selected from the group consisting of: dentaldevices, spinal devices, prosthetic devices; and transcutaneous devices.15. The method of claim 1, wherein the porous metal article has a poresize substantially the similar to the size of the extractableparticulate used to form the pore.
 16. The method of claim 15, whereinthe pore size is varied by changing the size of the extractableparticulates.
 17. The method of claim 1, wherein the porous metalarticle has a pore shape substantially similar to the shape of theextractable particulate used to form the pore.
 18. The method of claim17, wherein the pore shape is varied by changing the shape of theextractable particulates.
 19. A method of making a porous metal article,the method comprising: a. blending a metal powder, a homogenizing aid,and a substantially solid particulate into a mixture; b. forming acomposite of the mixture; c. removing the particulate and homogenizingaid from the composite through dissolution in a fluid; and d. sinteringthe composite following said removing.
 20. The method of claim 19,wherein the composite is formed onto a substrate.
 21. The method ofclaim 19, wherein the step of forming the composite further comprisescompacting.
 22. The method of claim 21, wherein the compacting stepfurther comprises cold isostatic pressing.
 23. The method of claim 19,wherein the homogenizing aid and particulate are water soluble, andwherein the fluid is water.
 24. A method of making a porous metalarticle, comprising: a. blending a metal powder, a homogenizing aid, andan extractable particulate into a mixture; b. densifying the mixture atleast partially using pressure in excess of 5,000 pounds-per-squareinch; and c. removing both the homogenizing aid and the extractableparticulate from the mixture with a fluid.
 25. The method of claim 24,wherein said densifying further comprises cold isostatic pressing. 26.The method of claim 24, further comprising sintering a composite of themixture after said removing.
 27. The method of claim 24, wherein thehomogenizing aid and extractable particulate are water soluble, and thefluid is water.
 28. A method of making a porous metal article havingnon-homogeneous porosity, the method comprising: a. blending a metalpowder and an extractable particulate to form a first composite; b.compacting the first composite with a second composite comprising asecond metal powder using pressure in excess of 5,000 pounds-per-squareinch such that the first composite and the second composite formsubstantially distinct regions of a green article; c. removing theextractable particulate from the green article with a fluid to form theporous metal article; and d. sintering the porous metal article.
 29. Themethod of claim 28, wherein said compacing further comprises coldisostatic pressing.
 30. The method claim 28 wherein the first compositefurther comprises a homogenizing aid that can be removed from the firstcomposite with the same fluid used to remove the extractableparticulate.
 31. The method of claim 28, further comprising processingthe first composite prior to compacting the first composite with thesecond composite.
 32. The method of claim 28, further comprisingprocessing the second composite prior to compacting the second compositewith the first composite.
 33. The method of claim 28, wherein the secondcomposite further comprises an extractable particulate.
 34. The methodof claim 28, wherein the second composite consists only of a metalpowder.
 35. A method of making a porous metal article, the methodcomprising: a. blending metal powder particles, a homogenizing aid, andan extractable particulate to form a composite; b. forming the compositeinto a green article by deforming the metal powder particles usingpressure in excess of 5,000 pounds-per-square inch; and c. removing thehomogenizing aid and the extractable particulate from the green article.36. The method of claim 35, wherein substantially all of thehomogenizing aid and the extractable particulate are removed bydissolving in a fluid.
 37. The method of claim 35, wherein the porousmetal article is further processed to form at least part of anorthopedic implant.
 38. The method of claim 37, wherein the orthopedicimplant further comprises a portion for interfacing a bone.
 39. Themethod of claim 35, wherein the porous metal article is furtherprocessed to form at least part of a device selected from the groupconsisting of: dental devices, spinal devices, prosthetic devices; andtranscutaneous devices.
 40. The method of claim 39, wherein the devicefurther comprises an interface portion, in relation to a bone, that isthe porous metal article.
 41. The method of claim 35, wherein the porousmetal article is further processed to form at least part of a filter.42. The method of claim 35, wherein the metal powder particles areselected from the group consisting of: titanium, tantalum, cobaltchrome, niobium, stainless steel, nickel, copper, aluminum, and alloysthereof.
 43. The method of claim 35, wherein the extractable particulateis selected from the group consisting of: salts, sorbates, inorganicmaterials, and organic materials.
 44. The method of claim 35, whereinthe extractable particulate has a diameter between about 100 and 2000micron.
 45. The method of claim 35, further comprising sintering thegreen article.
 46. The method of claim 35, wherein said formingcomprises cold isostatic pressing.
 47. The method of claim 35, whereinthe homogenizing aid and the extractible particulate are water soluble,and wherein the fluid is water.